Mix normal texture with noise texture to create flat patches
You could use the following material set up to achieve the desired effect :



This is using a Bump node instead of the Displacement socket as it is more easily configurable. Using Displacement (True Displacement) can result in gaps in your surface - especially if there are hard edges in the texture (such as where the surface transitions between brick and plaster). The Color Ramp allows the noise texture to be modified to produce the desired patches and this passes on to a Maximum node so that the flat plaster gives way to brick as the noise texture drops low. This will mean that the White areas will be the plaster rather than the Black in your example.

The Bump node is used to apply the texture to the surface Normal and this gives the impression of displacement rather than actually displacing the surface.

The Invert node is there simply to invert the brick texture I am using as my particular texture has light mortar and dark bricks. I am using the raw colour data to determine the depth and this is not ideal (it's under the assumption that lighter colours (the mortal) are set back more than the bricks). The inversion ensures that the mortar is inset rather than protruding.

You can adjust the plaster by varying the Color Ramp - this can adjust the amount of plaster/holes as well as changing the hardness of the edges by making the transition between Black and While sharper or softer. The thickness of the plaster can be varied by reducing the brightness of the White.

The Bump node can be amended to change the height/strength of the bump effect and this will affect the thickness of the plaster as well as the strength of the brick texture. The Normal from the Bump node feeds into the shader Normal.

Here's the brick texture I used. It isn't ideal as it has dark shadows which affect the bump mapping :



Tweaking the Noise distortion and adding colour based on the presence of plaster can produce the following result :



 

Or adjust it for a more subtle effect :